The present invention is an improved process for contacting hydrogen gas and tetrachlorosilane in a reactor comprising a pressurizable shell having located therein a reaction vessel forming a substantially closed inner chamber for reacting the hydrogen gas with the tetrachlorosilane. The improvement comprises feeding to an outer chamber between the pressurizable shell and the reaction vessel a gas or gaseous mixture having a chlorine to silicon molar ratio greater than about 3.5. The improvement reduces the concentration of hydrogen and tetrachlorosilane in the outer chamber that results from leakage of these gases from the substantially closed inner chamber and the detrimental reactions associated with such leakage on structural elements and performance of the reactor.
In a typical process for producing hyperpure semiconductor-grade silicon, trichlorosilane gas is reduced in the presence of hydrogen and deposited onto a heated element. A significant portion of the trichlorosilane gas fed to such a process is de-hydrogenated to form by-product tetrachlorosilane. It is desirable to convert this by-product tetrachlorosilane back into trichlorosilane which can be recycled to the deposition process.
Rogers, U.S. Pat. No. 3,933,985, issued Jan. 20, 1976, describes a process for converting tetrachlorosilane to trichlorosilane. The process involves passing hydrogen and tetrachlorosilane vapors through a reaction chamber held at a temperature of between 900.degree. C. and 1200.degree. C.
Weigert et al., U.S. Pat. No. 4,217,334 issued Aug. 12, 1980, describe an improved process for converting tetrachlorosilane to trichlorosilane. The process involves reacting tetrachlorosilane with hydrogen at a temperature of 600.degree. C. to 1200.degree. C., the tetrachlorosilane and hydrogen mixture having a molar composition between 1:1 and 1:50 in equilibrium with trichlorosilane and hydrogen chloride, and quickly quenching the reacted mixture to below 300.degree. C. The process used by Weigert et al. was conducted in what is described as a gas-tight tube constructed from carbon.
Reactors for the hydrogenation of tetrachlorosilane by hydrogen gas must be able to withstand high temperatures and the corrosive nature of materials such as chlorosilanes and hydrogen chloride gas formed during the hydrogenation process. Therefore, carbon based materials, including carbon, graphite, carbon fiber composites and the like are typically used inside the reactor. The carbon based materials can be used, for example, as heat insulating materials, as heating elements, and to form a vessel within the reactor in which to react the hydrogen gas and tetrachlorosilane.
In such reactors it is typically not possible to entirely confine the feed hydrogen gas and tetrachlorosilane to the reaction chamber. These gases leak through seals and joints in the reactor into surrounding spaces containing insulation materials and other structural elements. When hydrogen gas contacts these structural elements a number of detrimental reactions are possible depending on the composition of the structural element and the temperature at the contact location. For example, at temperatures of about 400.degree. C. to 1000.degree. C., hydrogen can react with carbon to form methane. This reaction can reduce carbon based materials such as electrodes, bridges, and insulation into a fine powder. The methanization reaction can also be a major source of carbon contamination in the trichlorosilane product. A second reaction which can take place in the reactor at temperatures above about 800.degree. C. in the presence of hydrogen and chlorosilanes is the conversion of carbon based materials into silicon carbide with the liberation of hydrogen chloride. This reaction can degrade the physical integrity of the carbon based-elements. The liberated hydrogen chloride can inhibit the production of trichlorosilane. A third reaction is the deposition of silicon on high-temperature elements within the reactor. In an atmosphere comprising hydrogen gas and tetrachlorosilane where the concentration of hydrogen gas is greater than about 85 mole percent, the tetrachlorosilane can be reduced to elemental silicon and deposited on high-temperature parts of the reactor. A buildup of silicon in the reactor can inhibit heat transfer within the reactor as well as make elements of the reactor brittle and difficult to disassemble. Also hydrogen gas has a high thermal conductivity and its presence in the space between the reaction vessel and the reactor shell can cause increased heat loss from the reactor and increased shell temperature in comparison to gases with lower thermoconductivity.
The present inventors have found that the detrimental actions associated with the present of hydrogen gas in contact with elements of the reactor outside the intended reaction zone can be reduced by feeding to the area of the reactor outside the reaction zone a gas or gaseous mixture having a chlorine to silicon molar ratio greater than about 3.5. A preferred feed gas is selected from a group consisting of tetrachlorosilane and hydrogen chloride.